I have both. Played Overwatch pretty heavy for a while but only played maybe 2-3 rounds of Battleborn before I just uninstalled it. Luckily Battleborn came from a cheap bundle so I ain't too mad about it sucking but I can understand why others are. They are not really that similar. They are compared because they are both cartoony shooters released around the same time but they are very different in gameplay. Overwatch is closer to a straight FPS with only cosmetic unlocks. Battleborn is a real FPS-MOBA mix with actual levelling up and power unlocks.

Overwatch generally wins the comparisons because it is easy to pick up and play. Once you know the characters and maps you just get better with the characters. People who are good at FPS will be good at Overwatch. Battleborn takes a while as you are going to grind it out unlocking things so you can play better. It relies as much on what you unlocked as how good you are at playing.

The overall hate probably comes from how hyped it was compared to how plain it is. It wasn't sold properly as it probably is more attune to WOW players sensibilities than say COD or BF even though it was sold like straight FPS players would love it.

Overwatch is crappy too now. They add one map and one character every few months, but there is not a lot of variety and each map usually only has one mode associated with it.